The invention relates to a method and system for changing the concentration of at least one material in a fluid of mixed materials, such as changing the concentration of a pollutant substance in a flowing stream of fluid, having a mixture of substances or gases, such as, but not limited to exhaust gases from an internal combustion engine. The method and apparatus include measuring the concentration of one material or gas in the flowing stream of fluid and utilizing the measurements for injecting material into the flowing stream to at least partly chemically convert the one material or another material in the stream to thus reduce the concentration of the one or the other material or pollutant substance.
If a translucent medium is penetrated by radiation, the intensity of the transmitted radiation is dependent on the medium, the materials admixed to the medium, and the frequency of the radiation. From these frequency-dependent attenuation values, an unknown substance in the medium can be identified or, in case of a known substance, its concentration can be determined in the measuring path.
DOS 2,525,375 discloses a process of this type for measuring the concentration of materials. In the conventional method, the light beam of a semiconductor diode with tunable emission wavelength is passed through a measuring medium and detected. In order to determine the concentration of a gaseous component to be detected in the measuring medium, the emission wavelength is periodically switched over between the absorption maximum and an adjacent minimum. When the center frequency of the radiation emitted by the diode lies on the flank of the absorption line, the detector receives an alternating signal which is passed on to a phase-sensitive detector. The output signal of the latter is a direct measure for the concentration of the gaseous component in the measuring medium.
Another process of this type for measuring the concentration of materials has been known from DOS 3,741,026. In this method, a laser is switched to and fro between two of its inherent resonant frequencies, one of the resonant frequencies lying in the absorption maximum and the other resonant frequency lying in the absorption minimum of the gas to be detected. The concentration of the gas to be analyzed is derived from the relationship of the intensity attenuation of the two resonant frequencies. The value of the radiation intensity transmitted in each case is stored in a sample+hold circuit. The values stored in the sample+hold circuit are processed with a computer circuit at each measuring cycle.
Another process of this type for measuring the concentration of materials has been described in U.S. Pat. No. 3,804,535. In this process, a light source is utilized transmitting alternatingly light of two different frequencies; this light is passed through a measuring medium with a gas component, the concentration of which component is to be determined. The amplifier for the detection of the transmitted light for the two frequencies is turned on only during transmission of the light in order to thereby reduce signal noise in the received signal.